Pneumatic radial tire having inner carcass ply cord tension smaller than the outer carcass ply cord tension

ABSTRACT

A pneumatic radial tire comprises a carcass comprising an outer carcass ply made of cords having a first heat-shrinkage-percentage K1 and an inner carcass ply is made of cords having a second heat-shrinkage-percentage K2 different from the first heat-shrinkage-percentage K1, and the cord tension of the inner carcass ply is smaller than the cord tension of the outer carcass ply. A method of making the tire comprises steps of making the outer carcass ply of cords having a first heat-shrinkage-percentage K1, making the inner carcass ply of cords having a second heat-shrinkage-percentage K2, wherein the first heat-shrinkage-percentage K1 at 150 degrees C. is 120 to 160% of the second heat-shrinkage-percentage K2 at 150 degrees C., and heating and vulcanizing a raw tire to heat-shrink the outer carcass ply cords in a larger degree than the inner carcass ply.

This application is a divisional of application Ser. No. 08/627,130,filed on Apr. 3, 1996, now U.S. Pat. No. 5,656,108, which is adivisional of application Ser. No. 08/465,013 filed Jun. 5, 1995, nowU.S. Pat. No. 5,338,572, the entire contents of which are herebyincorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a pneumatic tire and a method of makingthe same, and more particularly to an improvement relating to thecarcass by which the road noise is reduced.

Recently, passenger cars strongly require its less noise generation andquietness during vehicle operation. Therefore, it is necessary todecrease the noise generated from tires and the transmission of thenoise from the tire to the inside of the car. Road noise is a type ofnoise heard in a car. Road noise has a peak in the low frequency rangeof around 250 Hz, which is almost the same as the resonance frequency ofpassenger cars. During running, the tread portion vibrates due to theroughness of the road surfaces, and the vibrations are transmittedthrough a suspension mechanism and amplified to about 250 Hz. Therefore,the inside of the car tends to resonate or vibrate which is veryuncomfortable for the driver. This is especially noticeable in a radialtire whose tread portion is reinforced with a stiff belt.

It is therefore necessary for reducing the road noise, to decrease thesound level of a peak with a frequency about 250 Hz.

In order to reduce the road noise, the tread rubber harness is decreasedto decrease the tread rigidity, or the tread rubber thickness isincreased to provide a cushion effect against the shock which the treadreceives, or a foam rubber material is adhered to the inner surface ofthe tire as a vibration damper layer.

In such conventional countermeasure, however, desirable results can notbe obtained. If the tread rigidity is decreased, the cornering forcedeteriorates and the steering stability is deteriorated. If the damperlayer is used, the ride comfort is impaired, and the tire loses itsweight balance about its rotational axis. Further, the manufacturingmethod increases in the number of the steps and the manufacturing costincreases.

SUMMARY OF THE INVENTION

It is therefore, an object of the present invention to provide apneumatic tire and a method of making the same, in which the noise soundlevel of around 250 Hz can be decreased to decrease road noise, withoutsuffering from the above-mentioned problems.

According to one aspect of the present invention, a pneumatic tirecomprises a tread portion, a pair of axially spaced bead portions with abead core therein, a pair of sidewall portions, a carcass comprising aninner ply and an outer ply each extending between the bead portions, anda belt disposed radially outside the carcass and inside the treadportion, wherein the outer carcass ply is made of cords having a firstheat-shrinkage-percentage K1 and the inner carcass ply is made of cordshaving a second heat-shrinkage-percentage K2 different from the firstheat-shrinkage-percentage so that the cord tension of the inner carcassply is smaller than the cord tension of the outer carcass ply.

According to another aspect of the present invention, a method of makingthe pneumatic tire comprises steps of building a raw tire by assemblingthe inner and outer carcass plies, and heating the raw tire in a mold tovulcanize the tire, wherein the outer carcass ply is made of cordshaving a first heat-shrinkage-percentage K1 and the inner carcass ply ismade of cords having a second heat-shrinkage-percentage K2 differentfrom the first heat-shrinkage-percentage K1, wherein the firstheat-shrinkage-percentage K1 at 150 degrees C. is 120 to 160% of thesecond heat-shrinkage-percentage K2 at 150 degrees C., whereby, in thefinished tire, the cord tension of the inner carcass ply is smaller thanthe cord tension of the outer carcass ply.

As a result, the resonance frequency of the sidewall portions is changedto a higher frequency than 250 Hz by the above-mentioned carcassconstruction, and the vibration transmitting percentage at 250 Hz isdecreased. Accordingly, the resonance of the car is controlled and theroad noise is reduced.

Further, as the heat-shrinkage-percentage of the inner carcass ply cordsis low, the finished tire is improved in dimensional stability andexhibits a good steering stability and flat spot resistance.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the present invention will now be explained accordingto the drawings.

FIG. 1 is a cross sectional view of a tire according to the presentinvention.

FIG. 2 is an enlarged cross sectional view of the bead portion thereofshowing an example of the carcass structure.

FIG. 3 is a schematic cross sectional view showing another example ofthe carcass structure.

FIG. 4 is a cross sectional view of a tire showing another example ofthe carcass structure.

FIG. 5 is a schematic cross sectional view showing still another exampleof the carcass structure.

FIG. 6 is a schematic cross sectional view showing yet another exampleof the carcass structure.

FIG. 7 is a graph showing the relationship between theheat-shrinkage-percentage ratio K1/K2 and the road noise.

FIGS. 8(a), (b) are diagrams for explaining crystal structures of thecarcass cord.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the drawings, the pneumatic radial tire 1 according to the presentinvention comprises a tread portion 2, a pair of sidewall portions 3each extending radially inwardly from each edge of the tread portion 2,a pair of axially spaced bead portions 4 each located at the inner endof each of the sidewall portions 3, a pair of bead cores 5 each disposedin each of the bead portions 4, a carcass 6 extending between the beadportions 4, surrounding the tire cavity K, a belt (7, 9) disposedradially outside the carcass 6 and inside the tread portion 2, and abead apex 8 disposed in each of the bead portions 4.

In FIGS. 1 and 4 is shown is a standard state of the tire mounted on astandard rim R and inflated to a standard inner pressure specified inJIS and the like.

The tire 1 is formed as a low aspect ratio passenger radial tire havingan aspect ratio H/W--the ratio of the tire section height H to the tiremaximum width W--of not more than 0.70, for example 0.65.

The bead apex 8 is made of a high rigidity rubber having a substantiallytriangular cross-sectional shape and extending radially outwardly fromthe bead core 5, so as to reinforce the bead portion 4 and the sidewalllower portion. Preferably, a rubber compound having a JIS-A hardness of80 to 95 degrees, and a complex elastic modulus E* of 300 to 600kg/sq.cm is used for the bead apex 8.

Each bead portion 4 is further provided with a chafer 12 extending alongthe bottom face of the bead portion 4 in order to prevent the beadportion 4 from being chafed by the rim R during running and from beingdamaged at the time of the mounting operation. The chafer 12 is extendedradially outwardly to cover the radially outer edge of a carcass turnupportion (described hereinafter).

The belt comprises a breaker belt 7 and further a band belt 9 in thisembodiment.

The breaker belt 7 comprises two plies 7A and 7B of high modulus beltcords, e.g. steel cords, aromatic polyamide fiber cords and the like.The plans are laid at a small angle of not more than 35 degrees withrespect to the tire equator C, so that the cords in one ply 7A are laidin a different direction from that in the other ply 7B so as to crosseach other.

The band belt 9 is disposed radially outside the breaker belt 7,covering at least the axially outer edges EO of the breaker belt 7, toprevent the breaker belt 7 from being lifted during high speed running.The band belt 9 in the example comprises a pair of axially spaced,narrow width, inner plies 9B disposed on the radially outside of thebreaker belt 7 to cover the edges EO thereof, and an outer ply 9Adisposed radially outside thereof to cover the overall width of thebreaker belt 7. Each of the band plies 9A and 9B is made of low moduluscords, for example nylon, having a diameter smaller than that of thebreaker cords and laid at an angle of 0 to 10 degrees with respect tothe tire equator C.

According to the present invention, the carcass 6 comprises two plies,an inner carcass ply (10, 20, 30) and an outer carcass ply (11, 21, 31)each extending from one of the bead portions 4 to the other bead portion4 through the sidewall portions 3 and tread portion 2. At least one ofthe inner and outer carcass plies is turned up around the bead cores 5to form a pair of turnup portions. The inner and outer carcass plies aremade of carcass cords arranged at an angle of from 70 to 90 degrees, inthis example nearly 90 degrees, with respect to the tire equator C. Forthe carcass cords, organic fiber cords, e.g. polyester, rayon, nylon andthe like are used.

In FIGS. 1 and 2, the inner carcass ply 10 is turned up around the beadcores 5 from the inside to the outside of the tire to form a pair ofturnup portions 10B and a toroidal main portion 10A therebetween,whereby the above-mentioned bead apex 8 is positioned between the mainportion 10A and turnup portion 10B. The outer carcass ply 11 is however,not turned up around the bead cores. Therefore, it consists of atoroidal main portion 11A disposed outside the main portion 10A of theinner carcass ply 10. The radially inner edge 11a of the main portion11A is secured between the axially outer surface of the bead apex 8 andthe axially inner surface of the turnup portion 10B.

FIG. 3 shows a modification of the outer carcass ply 11, in which itsradially inner portion extends along the axially inner surface of thebead apex 8, and the inner edge 11a thereof is secured between the beadapex 8 and the main portion 10A.

Thus, the radially inner edge portions of the main portions 10A and 11Aof the inner and outer carcass plies 10 and 11 are tightly secured tothe bead cores 5 in the bead portions 4.

FIG. 4 shows still another example of the carcass 6, which comprises aninner ply 20 and an outer carcass ply 21. The outer carcass ply 21 isturned up around the bead cores 5 from the inside to the outside of thetire to form a pair of turnup portions 21B and a toroidal main portion21A therebetween, whereby the bead apex 8 is positioned between the mainportion 21A and turnup portion 21B. The inner carcass ply 20 is notturned up around the bead cores. Therefore, it consists of a toroidalmain portion 20A extending along and adjacent to the inner surface ofthe main portion 21A. The inner edges 20a thereof terminate at almostthe same height as the radially inner end of the bead core 5.

FIG. 5 shows a modification of the inner carcass ply 20, in which theinner carcass ply 20 is also turned up around the bead cores 5 so as toform a turnup portion 20B in the same way as the outer carcass ply 21.In this case, the turnup portion 20B is preferably extended radiallyoutwardly to completely cover the turnup portion 21B of the outercarcass ply 21.

FIG. 6 shows a modification of the carcass 6 shown in FIGS. 1 and 2. Inthis example, the inner carcass ply 30 comprises a main portion 30A anda pair of turnup portions 30B turned up around the bead cores 5 from theaxially inside to the outside. The outer carcass ply 31 is not turnedup, and the radially inner edge 31a is disposed axially outside of theturnup portion 30B of the inner carcass ply 30, and terminates near thebead core 5.

In any case, the radially inner edges 11a (FIGS. 1-3), 20a (FIG. 4) and31a (FIG. 6) terminate within a range Y between the radially inner andouter ends of the bead core. As a result, the sharing stressconcentration on the inner edge when the bead is deformed, can beprevented.

On the other hand, the radially outer edges of the turnup portions 10B(FIGS. 1-3), 21B (FIG. 4), 20B-21B (FIG. 5) and 30B (FIG. 6) terminateat a height Hm radially outward of the radially outer end of the beadcore 5 but radially inward of the radially outer edge of the flanges ofthe rim R, to thereby avoid the sharing stress concentration on theedge.

According to the invention, the cord tension of the outer carcass ply isset to be larger than the cord tension of the inner carcass ply in thefinished tire.

To achieve this, in the process of building a raw tire, organic cordshaving different heat-shrinkage-percentages K1 and K2 are used to makethe raw carcass plies. Then, to vulcanize the raw materials, the tire isput into a mold and heated while pressurizing the inside of the tire.

In order to practice such processes, the nowadays widely used tiremaking methods can be utilized. If explained briefly, such tire makingmethods comprise the following steps or processes: the raw carcass pliesare wound around a cylindrical drum's surface; the bead cores and beadapexes are disposed thereon while providing a space therebetween anddecreasing the space between the bead cores, the drum is expanded sothat the carcass has a toroidal shape; on the crown portion of thecarcass, the breaker plies and band plies are disposed; and a rubbertread, sidewalls, beads and other layers are disposed thereon to form araw tire. Then the raw tire is put in a mold and vulcanized.

According to the present invention, the cords for the outer carcass ply(11, 21, 31) have a first heat-shrinkage-percentage K1 and the cords forthe inner carcass ply (10, 20, 30) have a secondheat-shrinkage-percentage K2 which is different from K1. That is, aplurality of organic cords having the second heat-shrinkage-percentageK2 are laid parallel with each other and embedded in topping rubber inthe form of a rubber sheet. This sheet is cut in a suitable size andwound around the above-explained drum as the raw inner carcass ply (10,20, 30). Similarly, a plurality of organic cords having the firstheat-shrinkage-percentage K1 are laid parallel with each other andembedded in topping rubber in the form of a rubber sheet and cut into asuitable size. This sheet is wound around the previously wound innercarcass ply as the raw outer carcass ply (11, 21, 31) so that the innercarcass ply cords cross the outer carcass ply cords at a small angle.

Thereafter, in the tire vulcanizing process, these raw plies are heatedand then cooled. As a result, through the vulcanizing process, the outercarcass ply cords are heat-shrunk to a larger degree than the innercarcass ply cords, and thus the outer carcass ply is greatly increasedin cord tension compared to the inner carcass ply in the finished tire.

Here, the first heat-shrinkage-percentage K1 is set in the range of from120 to 160%, more preferably 130 to 150%, of the secondheat-shrinkage-percentage K2. If K1 is less than 120% of K2, the noisereduction is insufficient. If K1 is more than 160% of K2, the tirerigidity decreases and the steering stability deteriorates, andfurthermore, the noise has a tendency to increase.

In order to maintain the dimensional stability of the finished tire, theheat-shrinkage-percentage K2 of the inner carcass ply is preferably setin the range of from 3.5 to 5.0%.

The above-mentioned heat-shrinkage-percentage is obtained as follows.First, the original length (x) of the cord is measured, and the cord isput in a 150 degrees C. atmosphere for 20 minutes without being loaded.Then the length (z) of the heat-shrunken cord is measured to find outthe heat shrinkage (y). (y=x-z). The heat-shrinkage-percentage iscalculated as (y/x)×100.

The above-mentioned difference between the heat-shrinkage-percentages K1and K2 can be provided as follows.

When the inner carcass ply is materially the same as the outer carcassply, the difference is provided by heat-shrinking both the inner andouter carcass ply cords prior to vulcanization under differentheat-treating conditions, e.g. heating time, temperature and the like,or alternatively, by heat-shrinking only the inner carcass ply cordsprior to vulcanization to decrease the heat-shrinkage-percentage K2.

Further, during treatment of the cord by dipping into resin or latex, bychanging the cord tension, the heat-shrinkage-percentage can be changed.If the cord tension is increased during the dip treatment, theheat-shrinkage during tire vulcanization is increased.

Furthermore, in case of the same material, the crystal structure of thecord is changed. For example, a regular polyester cord and ahigh-modulus polyester cord are used.

FIGS. 8(a) and (b) diagrammatically show the crystal structure of aregular polyester and that of a high modulus polyester. In FIGS. 8(a)and (b), 15 is the crystal region, and 16 is the amorphous region.Between the regular modulus polyester and the high modulus polyester,the size, percentage, and orientation of such regions differ. The highmodulus polyester exhibits a low heat-shrinkage-percentage, and theregular polyester exhibits a higher heat-shrinkage-percentage.

Still further, the different cord materials may be used. For example, anylon fiber cord and a polyester fiber cord can be used.

However, materially the same cords are preferably used in view of tireperformances.

FIG. 7 shows a relationship between the ratio K1/K2 and the road noise,obtained through a test. In the test, test tires of size 215/65R15mounted on a standard rim of 15×6 1/2JJ and inflated to a pressure of2.0 ksc were installed on the front and rear wheels of a FR-typepassenger car. Then, the test car was run on a rough asphalt road at aconstant speed of 50 km/h, and the noise was measured in the car nearthe driver's ear on the inner side of the car.

As shown in FIG. 7, as the ratio K1/K2 increases, overall noisedecreases, and the 250 Hz noise especially decreases when the ratioK1/K2 is in the range of from 1.20 to 1.60.

Test tires of a size 215/65R15 having the same structure as shown inFIG. 1 except for the carcass constructions, were prepared and testedfor road noise, steering performance, ride comfort, flat spot resistanceand tire uniformity. The specifications and test results are shown inTable 1 and Table 2.

The road noise was measured in the same way as in the noise testmentioned in relation to FIG. 7. In Tables 1 and 2, the noise in eachcarcass construction (FIGS. 1-2, 6, 7, 8 or 3) is expressed in decibelas the difference from a conventional arrangement (K1=K2).

The ride comfort and steering stability were evaluated by the driverwhile running on a dry asphalt road.

The flat spot resistance was evaluated by the driver while running at 60km/h for 1 km after the test tire (size:215/65R15), mounted on astandard rim (size:15×6 1/2JJ) and inflated to a standard inner pressure(2.0 ksc) and loaded with 500 kg, was not permitted to move for fivedays.

As for tire uniformity, the radial force variation (RFV) was measuredaccording to JASO-C607 (Test method for automobile tire uniformity).

In the case where both the inner and outer carcass plies were made ofmaterially the same regular polyester cords, in order to provide adifference in the heat-shrinkage-percentages, the cords underwentdifferent heat treatments before vulcanizing the tire.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

                                      TABLE 1    __________________________________________________________________________                   Ex.a1                       Ex.a2                           Ex.a3                               Ex.a4                                   Ex.a5                                       Conven.a                                            Ref.a1                                                Ref.a2    __________________________________________________________________________    CARCASS    Construction   FIG. 1, 2    Ha (aa)        20    Hd (aa)        6    Inner carcass ply    No. of ply     1   1   1   1   1   1    1   1    Ply type *1    TUP TUP TUP TUP TUP TUP  TUP TUP    Cord material *2                   HM-PE                       HM-PE                           HM-PE                               HM-PE                                   HM-PE                                       HM-PE                                            HM-PE                                                RG-PE                   1500d/2                       1500d/2                           1500d/2                               1500d/2                                   1500d/2                                       1500d/2                                            1500d/2                                                1500d/2    Finished cord count (ends/5 cm)                   53.5                       53.5                           53.5                               53.5                                   53.5                                       53.5 53.5                                                53.5    Cord angle (deg.)                   90  90  90  90  90  90   90  90    Heat shrinkage percentage K2(%)                   4.7 4.7 4.7 4.7 4.7 4.7  4.7 7.0    Outer carcass ply    No. of ply     1   1   1   1   1   1    1   1    Ply type *1    DWN DWN DWN DWN DWN DWN  DWN DWN    Cord material *2                   RG-PE                       RG-PE                           RG-PE                               RG-PE                                   RG-PE                                       HM-PE                                            RG-PE                                                RG-PE                   1500d/2                       1500d/2                           1500d/2                               1500d/2                                   1500d/2                                       1500d/2                                            1500d/2                                                1500d/2    Finished cord count (ends/5 cm)                   53.5                       53.5                           53.5                               53.5                                   53.5                                       53.5 53.5                                                53.5    Cord angle (deg.)                   90  90  90  90  90  90   90  90    Heat shrinkage perentage K2(%)                   5.7 6.0 6.5 7.0 7.5 4.7  8.0 7.0    K1/K2 ratio    1.21                       1.28                           1.38                               1.49                                   1.59                                       1.0  1.70                                                1.0    BELT    No. of ply     2   2   2   2   2   2    2   2    Cord material  steel                       steel                           steel                               steel                                   steel                                       steel                                            steel                                                steel    Finished cord count (ends/5 cm)                   34  34  34  34  34  34   34  34    Finished cord angle (deg.)                   24  24  24  24  24  24   24  24    Test Result *3    Noise    Overall        -0.2                       -0.6                           -1.0                               -1.2                                   -1.2                                       0    -1.3                                                -1.3    125 Hz         -0.2                       -0.5                           -0.8                               -0.9                                   -0.9                                       0    -1.0                                                -0.8    250 Hz         -0.4                       -1.0                           -1.5                               -1.7                                   -1.8                                       0    -1.9                                                -1.8    Steering stability                   A   A   A   A   B   A    C   A    Ride comfort   A   A   A   A   A   A    B   A    Flat spot resistance                   A   A   A   A   A   A    A   B    Tire uniformity                   A   A   A   A   A   A    A   A    __________________________________________________________________________     1) TUP = turned up DFK = not turned up     2) HMPE = High modulus polyester RGPE = Regular polyester     3) A = Good, B = a little no good, C = No good

                                      TABLE 2    __________________________________________________________________________                   Ex.b                       Conven.b                            Ex.c                                Conven.c                                     Ex.d                                         Conven.d                                              Ex.e                                                  Conven.e    __________________________________________________________________________    CARCASS    Construction   FIG. 4   FIG. 5   FIG. 6   FIG. 3    Ha (aa)        20       Ha2 = 20, Ha1 = 10                                     20       20    Hd (aa)        6        --       6        6    Inner carcass ply    No. of ply     1   1    1   1    1   1    1   1    Ply type *1    TUP TUP  TUP TUP  TUP TUP  TUP TUP    Cord material *2                   HM-PE                       HM-PE                            HM-PE                                HM-PE                                     HM-PE                                         HM-PE                                              HM-PE                                                  HM-PE                   1500d/2                       1500d/2                            1500d/2                                1500d/2                                     1500d/2                                         1500d/2                                              1500d/2                                                  1500d/2    Finished cord count (ends/5 cm)                   53.5                       53.5 53.5                                53.5 53.5                                         53.5 53.5                                                  53.5    Cord angle (deg.)                   90  90   90  90   90  90   90  90    Heat shrinkage percentage K2(%)                   4.7 4.7  4.7 4.7  4.7 4.7  4.7 7.0    Outer carcass ply    No. of ply     1   1    1   1    1   1    1   1    Ply type *1    DWN DWN  DWN DWN  DWN DWN  DWN DWN    Cord material *2                   RG-PE                       HM-PE                            RG-PE                                HM-PE                                     RG-PE                                         HM-PE                                              RG-PE                                                  HM-PE                   1500d/2                       1500d/2                            1500d/2                                1500d/2                                     1500d/2                                         1500d/2                                              1500d/2                                                  1500d/2    Finished cord count (ends/5 cm)                   53.5                       53.5 53.5                                53.5 53.5                                         53.5 53.5                                                  53.5    Cord angle (deg.)                   90  90   90  90   90  90   90  90    Heat shrinkage perentage K2(%)                   6.5 4.7  6.5 4.7  6.5 4.7  6.5 4.7    K1/K2 ratio    1.38                       1.0  1.38                                1.0  1.38                                         1.0  1.38                                                  1.0    BELT    No. of ply     2   2    2   2    2   2    2   2    Cord material  steel                       steel                            steel                                steel                                     steel                                         steel                                              steel                                                  steel    Finished cord count (ends/5 cm)                   34  34   34  34   34  34   34  34    Finished cord angle (deg.)                   24  24   24  24   24  24   24  24    Test Result *3    Noise    Overall        -0.9                       0    -1.1                                0    -0.2                                         0    -0.9                                                  0    125 Hz         -0.7                       0    -0.8                                0    -0.1                                         0    -0.7                                                  0    250 Hz         -1.4                       0    -1.6                                0    -0.4                                         0    -1.5                                                  0    Steering stability                   A   A    A   A    A   A    A   A    Ride comfort   A   A    A   A    A   A    B   A    Flat spot resistance                   A   A    A   A    A   A    A   A    Tire uniformity                   A to B                       A    B   A    A   A    A   A    __________________________________________________________________________     1) TUP = turned up DWN = not turned up     2) HMPE = High modulus polyester RGPE = Regular polyester     3) A = Good, B = a little no good, C = No good

We claim:
 1. A pneumatic tire which exhibits reduced road noise whichcomprisesa tread portion, a pair of axially spaced bead portions, eachwith a bead core therein, a pair of sidewall portions, a carcasscomprising an inner ply and an outer ply, each extending between thebead portions, a belt disposed radially outside the carcass and insidethe tread portion, whereinthe outer carcass ply is made of cords havinga first heat-shrinkage-percentage K1 and the inner carcass ply is madeof cords having a second heat-shrinkage-percentage K2 whereby the firstheat-shrinkage-percentage K1 is 130 to 150% of the secondheat-shrinkage-percentage K2 so that the cord tension of the innercarcass ply is smaller than the cord tension of the outer carcass ply.2. The pneumatic tire according to claim 1, whereinthe inner carcass plyis turned up around the bead cores from the axially inside to outsidethereof to form a pair of turnup portions, and the outer carcass ply isnot turned up around the bead cores so that each of the radially inneredges thereof terminates near the axially inside or outside of therespective bead core.
 3. The pneumatic tire according to claim 2,whereinthe radially inner edges of the outer carcass ply are disposed onthe axially inside of the turnup portions of the inner carcass ply. 4.The pneumatic tire according to claim 3, whereinsaid cords having thefirst heat-shrinkage-percentage K1 and said cords having the secondheat-shrinkage-percentage K2 are shrunk by heating during vulcanizationof process for the tire.
 5. The pneumatic tire according to claim 2,whereineach of the radially inner edges of the outer carcass ply isdisposed on the axially outside of one of the turnup portions of theinner carcass ply.
 6. The pneumatic tire according to claim 5,whereinsaid cords having the first heat-shrinkage-percentage K1 and saidcords having the second heat-shrinkage-percentage K2 are shrunk byheating during vulcanization of process for the tire.
 7. The pneumatictire according to claim 2, whereinsaid cords having the firstheat-shrinkage-percentage K1 and said cords having the secondheat-shrinkage-percentage K2 are shrunk by heating during vulcanizationof the tire.
 8. The pneumatic tire of claim 1 wherein theheat-shrinkage-percentage K2 of the inner carcass ply cords is from 3.5to 5.0%.
 9. A pneumatic tire which exhibits reduced road noise whichcomprisesa tread portion, a pair of axially spaced bead portions, eachwith a bead core therein, a pair of sidewall portions, a carcasscomprising an inner ply and an outer ply, each extending between thebead portions, the inner carcass ply being turned up around the beadcores from the axially inside to outside thereof to form a pair ofturnup portions, the outer carcass ply not being turned up around thebead cores so that each of the radially inner edges thereof terminatesnear the axially inside or outside of the respective bead core, a beltdisposed radially outside the carcass and inside the tread portion,whereineach of the radially inner edges of the outer carcass ply isdisposed between the inner carcass ply turnup portion and the bead corein each of the bead portions, and the outer carcass ply is made of cordshaving a first heat-shrinkage-percentage K1 and the inner carcass ply ismade of cords having a second heat-shrinkage-percentage K2 whereby thefirst heat-shrinkage-percentage K1 is 130 to 150% of the secondheat-shrinkage-percentage K2 so that the cord tension of the innercarcass ply is smaller than the cord tension of the outer carcass ply.